1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED), and more specifically, to an OLED structured to increase light extraction efficiency.
2. Discussion of Related Art
In a typical organic light emitting diode (OLED), holes emitted from an anode combine with electrons emitted from a cathode in an organic emission layer (EML) formed between the anode and the cathode to generate excitons, and the excitons may recombine to emit light.
An OLED, which is an emissive device, has a wide viewing angle, a fast response speed, and high color reproduction characteristics so that the OLED is widely applied to display devices. Also, a vast amount of research is being conducted on light apparatuses to which OLEDs are applied.
An OLED may be configured to emit red (R), green (G), and blue (B) light or white light. In general, a white OLED may be applied to a lighting apparatus. An OLED applied to a lighting apparatus should have a higher luminance than an OLED applied to a typical display device.
In other words, an OLED having high light extraction efficiency is necessarily required to apply the OLED to a lighting apparatus. However, in a conventional OLED, at least 70% of light emitted by an organic EML may not be externally emitted out of the OLED but totally reflected in the OLED device.
The above-described problem of the conventional OLED will now be described with reference to FIG. 1.
FIG. 1 is a cross-sectional view illustrating a problem caused by total reflection of light in a conventional OLED. Although only the problem of a bottom-emitting OLED shown in FIG. 1 will be described for brevity, the description of the problem may be applied likewise to a top-emitting OLED or a double-sided-emitting OLED.
Referring to FIG. 1, a conventional OLED includes a transparent electrode 140a, an organic EML 150, a reflective electrode 140b, and an encapsulation substrate 160 that are sequentially stacked on a transparent substrate 110. For brevity, it is assumed that, as in a conventional OLED, the transparent electrode 140a and the organic EML 150 have the same refractive index, the transparent substrate 110 has a refractive index n1 lower than that of the transparent electrode 140a, a bottom surface of the transparent substrate 110 is in contact with the air, and the refractive index n1 of the transparent substrate 110 is higher than a refractive index n2 of the air.
In order to explain the problem of total reflection, FIG. 1 illustrates two light paths 201 and 202 which light emitted from the organic EML 150. Meanwhile, Snell's law that expresses a total reflection critical angle is given by Equation 1.
                                          sin            ⁢                                                  ⁢                          θ              C                                =                                    n              2                                      n              1                                      ,                            (        1        )            where θc denotes the total reflection critical angle at an interface between a first medium with a higher refractive index and a second medium with a lower refractive index when light travels from the first medium to the second medium, n1 denotes the refractive index of the first medium, and n2 denotes the refractive index of the second medium.
According to Equation 1, when an incidence angle is lower than the total reflection critical angle θc, light may travel toward the second medium without causing total reflection, while when the incidence angle is higher than the total reflection critical angle θc, light may be totally reflected toward the first medium.
Referring to FIG. 1, light emitted along the first light path 201 may be incident at an incident angle θ1 at an interface between the transparent substrate 110 and the air, while light emitted along the second light path 202 may be incident at an incidence angle θ2 at an interface between the transparent substrate 110 and the air.
The light emitted along the first light path 201 may not be totally reflected by the transparent substrate 110 but externally emitted because the incidence angle θ1 is smaller than the total reflection critical angle θc. However, the light emitted along the second light path 202 may be totally reflected by the transparent substrate 110 and wave-guided along the transparent substrate 110 because the incidence angle θ2 is greater than the total reflection critical angle θc.
Thus, the light extraction efficiency of a conventional OLED may be degraded due to its total reflection characteristics.
In order to solve the problem, various conventional methods have been attempted. However, the methods may substantially have low light extraction efficiency, involve complicated processes, require high fabrication costs, and shorten the lifetime of an organic EML.
Accordingly, it is necessary to develop a method of fabricating an OLED with improved light extraction efficiency at low cost using a simple process.